What is the name of the red plactic screw in the picture? It's from a toilets flushing mechanism. 
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I've never seen this type of flush valve. It appears that the water connection to the tank is at the top of the tank on the side rather than underneath as it usually is. Having the water enter above the level of water in the tank would prevent a gravity leak. – Jim Stewart Aug 29 '18 at 16:08
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I would call it the float height adjustment screw, possibly tank water level height adjustment screw.
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Older designed that had a more-or-less horizontal mechanism, that utilized a ball for a float, the connecting device was often called the float rod.
Here they call it a "link", but yours also performs the adjustment function.
